Cold sealable materials are known for packaging products that would be adversely affected by exposure to heat, such as might occur during heat sealing of a package, after the product has been placed therein. Candy bars with an external chocolate coating are an example of such a product.
Previously used cold seal packaging materials typically use one of two structures. One structure includes a layer of white oriented polypropylene (“OPP”) material, with ink printing on the outside, covered by a layer of lacquer, and a rubber latex cold seal coating on the inside. Another structure comprises a laminate of a white OPP ply and a clear OPP ply, bonded together by an adhesive, with a rubber latex cold seal coating on the exposed, inside, face of the white OPP ply. The clear OPP ply is reverse printed. Both of these structures present a number of problems. With the laminated structure, when the material is stored in rolls, the cold seal coating lies against the clear OPP outer ply, and it is difficult to prevent the cold seal coating from offsetting onto the outer ply when the material is unrolled. Special measures can be taken when applying the coating to ensure that the coating bonds securely to the white OPP ply, so that it will remain attached to that ply and not offset onto the outer ply. However, such measures increase the cost and complication of manufacture. The lacquer in the single-ply structure can be formulated to act as a release layer, but previously used lacquers often contain mobile additives. Such additives may poison the cold seal coating while the packaging material is stored in rolls and the cold seal coating is in contact with the lacquer, or may migrate through the packaging and contaminate the contents in use. Such lacquers are also not very stable, and have a limited life, because of the loss of volatile or migratory components.
Another problem with known cold seal packaging materials is associated with the cold seal itself. The package is sealed by aligning and contacting cold seal cohesive materials, which have been pattern applied to opposing surfaces of the package. Unfortunately, when a user pulls the opposing surfaces apart to open the package, the seal can fail at the interface between the two cohesive materials. The failure between the two cohesive materials leaves the cohesive substantially in tact on both of the opposed surfaces. This leaves open the possibility of a tamperer opening the package, tampering with the contents disposed therein, and then re-contacting the intact cohesive materials to again seal the package. Such tampering may be very difficult, if not impossible, for an unwary consumer to detect.